1. Field of the Invention
This invention relates to electrostatically-actuated light modulators and more specifically to a method of fabricating a micromirror faceplate using a combination of flat-panel manufacturing and MicroElectroMechanical Systems (MEMS) fabrication techniques.
2. Description of the Related Art
In an electrostatically-actuated light modulator, a beam of light is directed towards a light valve target that, in response to a video addressing signal, imparts a modulation onto the beam in proportion to the amplitude of the deflection of the individual reflective elements, e.g. a reflective thin-film or an array of micromirrors. The amplitude or phase modulated beam is then passed through projection optics to form the image. The target produces attractive electrostatic forces between the underlying substrate and the individual reflective elements that pull them inward toward the substrate. The amplitude of deflection corresponds to the pixel intensity in the video signal. It is well known that optical performance of the light modulator is closely tied to deflection range, electrostatic instability and resolution.
In the late 1960s, RCA developed a new Schlieren light valve that used a high energy scanning electron beam in a vacuum to address a thin metal film that is stretched over a support grid in close proximity to a glass substrate, which is described in J. A. van Raalte, "A New Schlieren Light Valve for Television Projection", Applied Optics Vol. 9, No. 10, (Oct. 1970), p. 2225. The electron beam penetrates the metal film and deposits charge on the glass substrate in proportion to the intensity of the video signal. The deposited charge produces an attractive force that deforms the metal film inward towards the substrate, which causes a portion of the reflected light to miss the stop, thereby increasing screen brightness until all the light reaches the screen. In actual operation, each pixel deforms parabolically so that light incident on the central portion of each pixel element is not deflected. This limits fill factor and optical efficiency. In addition, deflection range is limited to about 20% to maintain parabolic deformation.
More recently Optron Systems, Inc., as described in Warde et al., U.S. Pat. No. 5,287,215, has developed a membrane light modulation system in which a charge transfer plate (CTP) couples charge from a scanning electron gun under vacuum through to potential wells in atmosphere. An array of insulating posts formed in or on the CTP support a deformable reflecting membrane that spans the wells. The CTP serves as a high-density multi-feedthrough vacuum-to-air interface that both decouples the electron beam interaction from the membrane and provides the structural support required to hold off atmospheric pressure. The vacuum-to-air interface allows the reflective membrane to be built and operated in air rather than a vacuum.
Warde's membrane light modulator is fabricated by either a) removing material from the CTP's feedthroughs to form an array of recessed wells or b) photolithographically defining insulating spacers on the CTP to define the recessed wells. A polymeric membrane is deposited on the CTP over the wells such that a reliable bond between the two dielectric surfaces is established due to Van der Waals forces. The fabrication of the CTP is described in detail in U.S. Pat. Nos. 4,794,296 and 4,863,759.
As detailed in FIGS. 24-30 and column 18, line 1 to column 23, line 28 of U.S. Pat. No. 4,863,759, the high spatial resolution charge transfer feedthrough plate production assembly consists essentially of a liquid metal extruder that receives a porous insulative substrate and operates at elevated temperatures to fill each of the pores with a conductive metal. Since the production assembly must assure high resolution parallel conductors that are not shorted together and provide a very high precision vacuum seal between the feedthroughs and the insulating substrate, the process requires specially designed equipment and processing techniques that are far more expensive and less reliable than standard photolithographic techniques.
In the early 1970s, Westinghouse Electric Corporation developed an electron gun addressed cantilever beam deformable mirror device, which is described in R. Thomas et al., "The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays," ED-22 IEEE Tran. Elec. Dev. 765 (1975) and U.S. Pat. Nos. 3,746,911, 3,886,310 and 3,896,338. The device is fabricated by growing a thermal silicon dioxide layer on a silicon-on-sapphire substrate. The oxide is patterned in a cloverleaf array of four centrally joined cantilever beams. The silicon is isotropically wet-etched until the oxide is undercut, leaving four oxide cantilever beams within each pixel supported by a central silicon support post. The cloverleaf array is then metallized with aluminum for reflectivity. The aluminum deposited on the sapphire substrate forms a reference grid electrode near the edges of the mirrors that is held at a d.c. bias. A field mesh is supported above the mirrors to collect any secondary electrons that are emitted from the mirrors in response to the incident primary electrons. The device is addressed by a low energy scanning electron beam that deposits a charge pattern on the cloverleaf beams, causing the beams to be deformed toward the reference grid electrode on the substrate by electrostatic actuation.
Texas Instruments has pioneered the development of the digital-mode light modulator with its digital micromirror device (DMD) that uses the pull-in problem to its advantage. The DMD employs a torsional micromirror that is fabricated on a SRAM integrated circuit and rocks back-and-forth between binary positions with the tips of the mirror being pulled down to the base electrodes. Time division multiplexing (TDM), created by rapidly rocking the mirror back-and-forth between its two positions, is used to establish different gray-levels. The electronics for implementing a TDM addressing scheme are much more complex and expensive than those required for analog modulation. Fabrication of the SRAM requires the normal 14 mask levels on a 4-5" wafer in a CMOS process. The fabrication of the 16 .mu.m.times.16 .mu.m micromirrors in a large array requires an additional 8 mask levels. The low yield from this complex processing results in a high unit cost. Furthermore, anti-stick coatings complicate the device and increase production costs significantly.